Abstract: A washer device for preventing looseness reduced in thickness, simplified in part configuration to lower cost, and capable of automatically compensating for looseness of a connection part, formed of a bolt and nut, due to aging, such as shrinkage of a connected member, so as to continuously and reliably preventing the looseness of the connection part. The washer device is for preventing looseness and includes a pair of washers, wherein at least one of the washers is a dome washer having a bolt insert hole. A circular elastic body having a power spring wound around the periphery is disposed in a gap between a spherical surface of the dome washer and an opposing surface, wherein the gap gradually narrows from the periphery toward the center. The power spring having an expanded diameter due to an expansion force caused by tightening is wound around in the gap like a snake by a restoring force.